The present invention relates to a base station, mobile station, communication system and reordering method thereof, and more particularly to a base station, mobile station, communication system and reordering method where packets, to which numbers indicating sequence are attached, are sent from a base station to a mobile station, and the packets are rearranged in order of the sequence number in the mobile station.
CDMA service based on the third generation method has begun in mobile communication systems, but a next generation mobile communication system (LTE: Long Term Evolution) which allows even faster communication is now under consideration at 3GPP (Third Generation Partnership Project) (see Non-patent Document 1). Here a decrease in transmission delay is a major issue, in addition to an increase in transmission rate.
In order to increase the transmission rate and decrease the transmission delay, an attempt to speed up the handover processing in comparison with the conventional systems, is being made in the LTE communication system. In the case of mobile communication, a base station that communicates with a mobile station is switched to another base station according to the receive state when the mobile station is moving during communication. This is referred to as handover and a base station in communication before handover is called a “handover source base station”, and a base station to be in communication after handover is called a handover target base station. Therefore speeding up the handover is absolutely necessary to implement high-speed/low delay communication. In the LTE communication system, which is based on a packet exchange system, hard handover is used for the handover. In the case of hard handover, a line between a mobile station and a handover target base station is connected after a line between the mobile station and a handover source base station is disconnected. Hard handover can be performed quickly by obtaining system information of the handover target base station immediately before executing handover, but transmission of the user data is interrupted during handover.
This means that in order to decrease transmission delay, it is important to decrease the transmission interruption state and to prevent loss of packets during the transmission interruption state. If packets are dropped during the transmission interruption state, the dropped packets are recovered by end-to-end packet retransmission, so transmission delay increases.
Therefore in the handover of the LTE communication system, standardized is a method in which a handover source base station transfers at least packets, among the data including control information and packets addressed to the mobile station, to a handover target base station (see Non-patent Document 2). However, whether the data is actually transferred or not depends on whether this function is installed.
FIG. 19 is a diagram depicting packet transference during handover. In (A) of FIG. 19, two base stations 1a and 1b are connected to a host station (e.g. access gateway) 2. A mobile station 4 exists in a cell 3a of the base station 1a, and is currently communicating with the base station 1a. In this state, if the mobile station 4 moves toward the base station 1b and enters a cell 3b, as shown in (B) of FIG. 19, handover is executed, and the communication base station of the mobile station switches from base station 1a to base station 1b. A base station in communication before handover is called a “handover source base station” (source base station), and a base station to be in communication after handover is called a “handover target base station” (target base station).
The handover source base station 1a stores packets, received from the host station 2, in an internal buffer, and sequentially sends the packets stored in the buffer to the mobile station 4. Because of this, when handover occurs, some stored packets remain in the buffer without being sent to the mobile station. In (B) of FIG. 19, packets n−2 to n, which were received before handover and are not yet sent to the mobile station, exist in the buffer, and these packets must be sent from the handover target base station 1b to the mobile station 4 after handover. Therefore when the handover sequence is executed, the handover source base station 1a transfers (forwards) the packets n−2 to n to the handover target base station 1b (this is referred to as forwarding). By using this forwarding method, in which the handover target base station 1b sends these packets to the mobile station 4 immediately after handover, packet transmission is not interrupted. Hence end-to-end packet retransmission is not required, and high-speed handover can be executed. The above mentioned n−2 to n are numbers which indicate a sequence of packets (sequence number).
FIG. 20 is a diagram depicting a handover of an LTE communication system, and FIG. 21 is a diagram depicting a handover procedure which is currently assumed in the LTE communication system.
The mobile station (UE) 4 notifies the handover source base station 1a that handover HO is necessary, using the Measurement Report (1. Measurement control).
The handover source base station 1a decides a target base station 1b based on the content of the Measurement Report (2. HO decision), and sends the handover request to this handover target base station 1b (3. HO request). HO means handover. At this time, the handover source base station 1a also sends information on the mobile station (e.g. mobile station ID and QoS (Quality of Service) information to the handover target base station). The handover target base station 1b executes call acceptance control based on this information (4. Call acceptance control).
If the handover target base station 1b admits acceptance of the mobile station, it returns handover response to the handover source base station (5. HO response). Then, the handover source base station 1a instructs handover to the mobile station 4 (6. HO instruction), thereafter starts transfer of data (packets) before or after this step (packet transfer: forwarding).
The mobile station 4 which received the handover instruction insures synchronization with the handover target base station 1b by L1/L2 signaling (7. Synchronization insuring), and when the synchronization is insured, the mobile station 4 sends the handover completion report to the handover target base station 1b (8. HO completion).
Thereby the handover target base station 1b sends the handover completion report to the host station 2 (9. HO completion). The host station 2 which received the handover completion report switches the packet transmission path from the handover source base station 1a to the handover target base station 1b (10. Path switching), and returns the HO completion response to the handover target base station 1b (11. HO completion response). By the HO completion response, the handover target base station 1b notifies the handover source base station 1a that the handover HO is completed (12. Resource release). Then the path between the handover source base station 1a and the host station 2 is cleared (13. Resource release).
If packet transference (forwarding) is generated during the execution of the above mentioned handover sequence, the transferred packets may be jumped over by packets which flow into the handover target base station 1b from the host station 2, and the sequence numbers may be out of order. If the handover target base station 1b transfers the packets to the mobile station 4 without correcting the order, the mobile station cannot receive the packets in correct order, whereby communication quality deteriorates and high quality communication before and after the handover cannot be implemented as a result.
Therefore in the LTE communication system, the packet sequence consistency between the base station and the mobile station is maintained by the following method. FIG. 22 is a diagram depicting the packet sequence consistency, where the handover target base station 1b maintains packet sequence consistency by transmitting the packets transferred from the handover source base station 1a with priority over the packets received from the host station. In other words, packets n−5 to n have been stored in the handover source base station 1a before handover, and handover is then generated, so the packets n−5 to n−3 are transferred to the handover target base station 1b and stored in the buffer BF. In the buffer BF of the handover target base station 1b, packets n+1 and n+2, received from the host station, have also been stored.
The handover target base station 1b, which stores the transferred packets n−5 to n−3 and the packets n+1 to n+2 that flowed into in the buffer from the host station, sends the packets n−5 to n−3 transferred from the handover source base station 1a first to the mobile station. Then if a delay exists in the transfer of the packets n−2 to n from the handover source base station 1a, the handover target base station 1b sends the packets n+1 to n+2 to the mobile station. The mobile station 4 executes processing to rearrange the received packets in the order of sequence numbers (reordering).
FIG. 23 is a diagram depicting the reordering processing of the mobile station. In FIG. 22, the mobile station 4 received the packets n−5 to n−3 in the order of sequence numbers, so the packets are sequentially transferred to the upper layer. At the point of receiving the packets n+1 to n+2, however, packets n−2 to n have not yet arrived. Therefore the mobile station 4 stores the packets n+1 to n+2 in the buffer BF1 until receiving these packets n−2 to n which have not yet arrived, and does not transfer these packets n+1 to n+2 to the upper layer. When the packets n−2 to n are received, the mobile station 4 transfers these packets and packets n+1 to n+2 sequentially to the upper layer. An upper limit is normally set for the packet wait time, and is measured by a timer of the mobile station, for example.
As mentioned above, in the case of handover in the LTE communication system, the essential technologies are the packet transfer (packet forwarding) from the handover source base station to the handover target base station and the packet reordering processing at the mobile station. The relationship of these functions will now be described in detail.
FIG. 24 is a diagram depicting a protocol configuration between the mobile station and the network. Between the mobile station and the network, at least a PDCP (Packet Data Convergence Protocol) layer, RLC (Radio Link Control) layer and lower layer (MAC layer/physical layer MAC/PHY) are installed. These layers are all installed in the mobile station, but are not always installed in a station at the network side. In the case of the example in FIG. 25, the PDCP layer is installed in the host station aGW2, and the RLC layer and the lower layer are installed in the base station 1. The system may be constructed such that all of the PDCP layer, RLC layer and lower layer are installed in the base station 1, and only simple functions, such as the packet routing function and the sequence number attaching function, are provided to aGW2.
In the case of the example in FIG. 25, data in the PDCP layer is exchanged between the mobile station 4 and aGW2, which is the host station, and data in the RLC layer is exchanged between the user terminal 4 and the base station 1.
In other words, data addressed to the mobile station flows from the upper layer (e.g. IP layer) to the PDCP layer first, and becomes PDCP SDU (Service Data Unit), and then header information (e.g. sequence numbers in PDCP layer) is added, and the PDCP PDU (Protocol Data Unit) is created.
The PDCP PDU is routed to the RLC layer, and becomes RLC SDU, and then header information (e.g. sequence number of RLC layer) is added, and RLC PDU is created. The RLC PDU arrives at the RLC layer of the mobile station via the processing in the lower layer. In the RLC layer, the header is removed and RLC SDU is reconstructed, then in the PDCP layer, the header of the PDCP PDU is removed, and PDCP SDU is created and routed to the upper layer of the mobile station.
In this protocol configuration, in the LTE communication system, packet transfer is executed in RLC SDU units or in PDCP SDU units, and reordering is executed in PDCP PDU units. Since RLC SDU and PDCP PDU are substantially the same data, they are simply referred to as “packets” in the present description, and it is assumed that a number of a packet described here indicates a sequence number of a PDCP PDU, unless otherwise specified.
FIG. 26 is a flow chart depicting an operation of the handover source base station device during handover.
When the handover source base station 1a receives the field strength of reception from the mobile station 4 via Measurement Report (step 101), the handover source base station 1a judges whether handover HO is necessary (step 102), and returns to the beginning if handover is unnecessary.
If it is decided that handover is necessary, the handover source base station 1a decides the handover target base station 1b based on the content of Measurement Report, and sends a handover request to the handover target base station 1b (step 103).
Then the handover source base station 1a receives a handover response which is sent from the handover target base station 1b (step 104), and judges whether data transference is executed (step 105). If the transference of the packets which are not sent to the mobile station and remain in the buffer is not executed, the handover source base station 1a sends HO instruction to the mobile station, and erases these packets (step 106). If the transference of the packets which are not sent to the mobile station and remain in the buffer is executed, on the other hand, the handover source base station 1a sends HO instruction to the mobile station, and forwards these packets to the handover target base station (step 107). The transference of the packets for a service which requires real-time processing, such as VoIP calling, is not executed, but is discarded. This is because discarding packets insures voice transmission and reception without delays. The transference of the packets for a service which requires high QoS is executed.
Then the handover source station 1a receives a resource release message which is sent from the handover target base station 1b (step 108), and executes a resource release (step 109).
FIG. 27 is a flow chart depicting operation of the handover target base station during handover.
When the handover target base station 1b receives an HO request (including mobile station ID and QoS information) from the handover source base station 1a (step 121), the handover target base station 1b performs call acceptance control based on this information, and judges whether acceptance of the mobile station is admitted or not (step 122). If not admitted, the handover target base station 1b performs post-processing (step 130), and ends handover control.
If acceptance of the mobile station, on the other hand, is admitted, the handover target base station 1b returns an HO response to the handover source base station 1a (step 123). Then the handover target base station 1b stores packets transferred from the handover source base station 1a in a buffer (step 124), and receives an HO completion report from the mobile station 4 (step 125). When the HO completion report is received, the handover target base station 1b sends the HO completion report to the host station 2 (step 126). When the handover completion report is received, the host station 2 switches the packet transmission path from the handover source base station 1a to the handover target base station 1b, and returns with the HO completion response to the handover target base station 1b(step 127). When the HO completion response is received from the host station 2, the handover target base station 1b starts sending packets forwarded from the handover target base station 1b preferentially to the mobile station, and starts sending packets received from the host station 2 to the mobile station after the above packets are sent (scheduling: step 128). The mobile target base station 1b also sends the resource release to the handover source base station 1a in parallel with step 128 (step 129), and performs post-processing (step 130), and ends handover control.
FIG. 28 is a flow chart depicting an operation of the mobile station during handover.
The mobile station 4 notifies the field strength of reception to the handover source base station using Measurement Report (step 151). The mobile station 4 then waits for an HO instruction from the handover source base station 1a, and if received (step 152), the mobile station 4 insures synchronization with the handover target base station 1b by L1/L2 signaling (step 153), and sends the handover completion report to the handover target base station 1b when synchronization is insured (step 154), then if packets are received from the handover target base station 1b, the mobile station executes reordering processing (steps 155 to 160).
In other words, when the lower layer packets are received from the handover target base station 1b, the control unit of the mobile station constructs RLC SDU, and transfers this RLC SDU to the reorder unit (step 155). The reorder unit checks whether the sequence number is continuous (step 156), and transfers this RLC SDU to the upper layer as PDCP SDU if the sequence numbers are continuous without missing any numbers (step 160). If the continuity of the sequence number of a RLC SDU ceases, on the other hand, the reorder unit stores RLC SDU (=PDC PDU)(step 157), and then checks whether the continuity of the sequence number is resumed by the received RLC SDU or not (step 158). If the continuity of the sequence numbers is resumed by the received RLC SDU, the reorder unit transfers this RLC SDU to the upper layer as PDCP SDU, and then transfers the stored RSC SDU (=PDCP PDU) to the upper layer (step 160).
In step 158, if the sequence numbers of the received RLC SDU are not continuous, the mobile station monitors whether a predetermined time has elapsed (step 159), and repeats the processing after step 157 if not, or transfers the stored PDCP PDU to the upper layer if elapsed, even if the sequence numbers are not continuous (step 160).
To execute packet transfer during handover in the LTE communication system, the following problem exists. That is, when handover is executed in the LTE communication system, the transference of the packets destined for the mobile station remaining in the handover source base station 1a is executed as mentioned above, thereby the packets are transferred to the handover target base station (forwarding). However, whether the transference of the packets executed during handover depends on whether this transference function is installed in the handover source base station.
As a result, even whether the handover source base station 1a did not execute packet transfer (packet forwarding), the mobile station 4, which was not notified, may judge that packet transfer was executed, and in such a case, after the handover, the reorder management unit of the mobile station must unnecessarily wait for the arrival of packets of which sequence numbers are continuous, until a predetermined time elapses. This results in that communication delays increase, and throughput deteriorates, and high quality communication quality cannot be maintained before and after the handover.
FIG. 29 shows an example of packets n−2 to n remaining in the handover source base station 1a. If these packets are not forwarded to the handover target base station 1b, the mobile station 4 must unnecessarily wait for the arrival of these packets n−2 to n, which will never be sent for a predetermined time after receiving packet n+1 from the handover target base station 1b. As a result, a communication delay occurs, and throughput of the entire system drops.
A first prior art that is available is a reassembling and reordering device, which restores packets before fragmentation from fragmented packets, and corrects reversal of the packet arrival sequence to recover the original sequence (Patent Document 1). However, this prior art relates to a reassembling and reordering device which restores packets before fragmentation from the packets fragmented in GTP tunnel (GTP: GPRS Tunneling Protocol) of GPRS (General Packet Radio Service), and corrects the reversal of the packet arrival sequence generated by the fragmentation and reassembles to the original sequence.
A second prior art that is available is a mobile communication system which realizes high-speed packet data transmission without generating data loss during handover between base stations in high-speed packet communication (Patent Document 2). In this mobile communication system, when a handover is generated between base stations in conformity with a communication state of a mobile station during high-speed packet communication between a base station and a mobile station, the handover source base station transfers the packet data to the handover target base station (forwarding).
However, neither the first nor the second prior arts are for suppressing an increase in communication delays or the deterioration of throughput due to the reordering of packets received from the handover target base station.
In view of the foregoing, it is an object of the present invention that the mobile station need not execute reordering at the packets in order of the sequence numbers even if they are not continuous, if the packets remaining in the handover source base station are not forwarded to the handover target base station (transference is not executed) when handover control is executed.
It is another object of the present invention that the mobile station executes reordering of the packets in order of the sequence numbers if the packets remaining in the handover source base station were forwarded to the handover target base station (transference was executed) when handover control is executed.    [Patent Document 1] Japanese Patent Application Laid-Open No. JP2004-135076A    [Patent Document 2] Japanese Patent Application Laid-Open No. JP2004-282652A    [Non-patent Document 1] 3GPP, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN),” TR25.913 V7.3.0, Release 7, March 2006    [Non-patent Document 2] 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN),” TS36.300, Release 8, Vol. 4.0, January 2007